A conventional television tube comprises an almost plane faceplate or screen of rectangular shape. The screen is furnished on its internal face with a mosaic of patches of phosphors or pixels which excited by an electron beam emit light which may be blue, green or red, depending on the phosphor excited.
An electron gun sealed in the envelope of the tube is directed towards the centre of the screen and makes it possible to emit the electron beam towards the various points of the screen through a perforated mask (or shadow mask). The electron gun makes it possible to focus the electron beam onto the internal face of the screen carrying the phosphors.
A deviating system placed around or on either side of the tube makes it possible to act on the direction of the electron beam so as to deviate its trajectory. Continual action of the deviating system thus allows horizontal and vertical scanning of the screen so as to explore the entire mosaic of phosphors.
Without deviation of the electron beam and with symmetric electrodes of the gun that create symmetric electric fields in the gun, the electron beam reaches the centre of the screen.
When the deviating system is acted on and the direction of the beam is deflected, the spot on the screen is deformed and the problem is all the more crucial as the beam is deflected towards the periphery of the screen or even towards the corners of the screen. In particular, in the case of a rectangular screen whose large dimension is horizontal, a horizontal deflection towards the left and right edges gives rise to a horizontally deformed spot. In the corners there is a vertically and horizontally combined deformation.
To remedy these defects, the art makes provision for electrodes made in the form of quadrupoles and controlled electrically in different ways in the vertical direction and in the horizontal direction, doing so in order to precompensate for the deformations of the beam just described.
The quadrupolar effects thus make it possible to achieve shape factors for the electron beams. These effects tend to counter the phenomena of distortion of shapes of beams created by the deviator in a situation of deviation towards the periphery of the screen and hence of deformation of size of spot on the screen. The shape factor must be dynamic as a function of the deviation of the beam.
The horizontal distortion of the electron beam towards the periphery of the screen is therefore the result of a magnetic deflection caused by the deviator deflecting the beam so as to effect the scanning of the screen, and associated with this deviator the action of an exit quadrupole in the gun. The combining of these effects results in a degradation of the horizontal resolution and a large improvement in the vertical resolution.
A quadrupole structure can comprise three electrodes composed of rectangular holes which make it possible to create the quadrupolar effect and also of circular holes which ensure the proper alignment of the various elements of the electron gun.
In electron guns intended to excite aligned colour pixels on the television screen, each electrode includes three holes allowing the processing and the transmission of three electron beams called the red, green and blue electron beams and intended to excite respectively the pixels of red, green and blue phosphors of the screen.
Moreover, so-called “high definition” guns can also comprise a second quadrupole whose effect is achieved via interdigitated elements called “interdigits” in the subsequent description. This quadrupole makes it possible to tailor the vertical size of the spots at the image edge. These “interdigits” are also used to correct defects related to the gun such as “MODEC” (deriving from the expression “beam deconvergence modulation”) by creating a dissymetry at the level of the structure of the holes. On the other hand this dissymetry becomes too great when working on “high-definition” guns.
To finalize the design of an electron gun, it is necessary to tailor the following parameters:
the difference between the place of impact on the screen of the beams outside the voltages V6 and Vf of nominal operation and the place of impact of the beams outside the voltages V6 and Vf of nominal operation at ±1000 volts commonly referred to as FRAT (deriving from the expression “focusing refraction alignment test”). It should be noted that nominal operation is the voltage pair V6 and Vf that makes it possible to focus the electron beams correctly at the centre of the screen.
The MODEC (deriving from the expression “modulation deconvergence”) which is the difference between the place of impact of the beams outside the voltages V6 and Vf of (nominal) operation and the place of impact of the beams outside the voltage V6+1000 volts and the nominal voltage Vf. It should be noted that V6 may be equal to Vf plus the modulation applied in a situation of deflection of the electron beams (V6=Vd+Vf).
The FODEC (deriving from the expression “focusing deconvergence”) which is the difference between the place of impact of the beams outside the voltages V6 and Vf of (nominal) operation and the place of impact of the beams outside the nominal voltage V6 and the voltage Vf+1000 volts.
As a general rule, the FRAT is corrected by design parameters for the BFR (deriving from the expression “beam formation region”) part of the electron gun.
For its part, the MODEC is corrected by a design parameter which occurs at the level of the “interdigits”. The “interdigits” form a quadrupolar structure making it possible to improve the vertical size on the edge of the screen. The “interdigits” (FIGS. 5 and 6) consist of two plates opposite one another spaced apart by a distance D.id (FIG. 6a) and each drilled with three holes such as 14, 15 and 16 corresponding to the three beams red, green and blue. Each of its holes is composed of two quasi quarters of a cylinder, such as A and B, that are symmetric in the X or Y axis. The quasi quarters of cylinders on the opposite holes are rotated by 90° in the Z axis so as to create the quadrupolar effect. On the outside holes, it is necessary to create a differential Diff (FIG. 6b) between these quarters of cylinders so as to ensure a zero MODEC.
As regards certain guns and more particularly guns designed for “high definition” televisions, the differential diff of the two quarters of cylinder of the outside holes is too big and could create a strong dissymmetry at the level of the shape of the beam.
The invention makes it possible to correct the MODEC without needing to dissymmetrize the heights of the opposite quarters of cylinders of the outside holes of the electrodes.
Its advantage is to recentre the beam without however disturbing the latter and it also makes it possible to preserve the conventional alignment of the various elements of the electron gun.